1. Field of the Invention
This invention relates to a resin composition with high storage stability. More particularly, it relates to a method for imparting storage stability to a resin composition which excels in formability as in terms of curability and workability and produces a cured article excelling in stabiliy to endure elevated temperatures and in mechanical strength.
2. Description of the Prior Art
As resins possessing high stability at elevated temperatures, various heat-resistant resins are cited which are represented by polyimide resins and polyamideimide resins. These heat-resistant resins, however, have important problems regarding formability as because they have high melting points and, therefore, necessitate use of a high temperature and a high pressure for molding, because they require to stand at a high temperature under a high pressure for a long time for curing, and because they require to be used as dissolved in special high-boiling solvents and, therefore, entail a step for the removal of the solvents by a protracted standing at an elevated temperature under application of pressure or under a vacuum. It is, therefore, extremely difficult to produce large formed articles from these heat-resistant resins or to produce formed articles of these resins continuously as by pultrusion molding or extrusion molding.
As resins excelling in formability in terms of curability, workability, etc., radical polymerization type resins such as epoxy (meth)acrylates derived from polyvalent phenol type epoxy resins such as bisphenol type epoxy resins or novolak type resins and (meth)acrylic acid or unsaturated polyesters have been known in the art. Generally, these resins are widely used in the form of vinyl ester resins or unsaturated polyester resins having incorporated therein a radically polymerizable cross-linking agent such as styrene. These resins, however, are not necessarily quite satisfactory in terms of thermal stability at elevated temperatures. This deficiency in thermal stability constitutes a serious hindrance to the development of applications for the resins. In the circumstances, the desirability of developing a resin excellent in thermal stability has been commanding the recognition of the industry.
We continued numerous studies for the purpose of developing a resin which is capable of producing cured articles of high stability at elevated temperatures without impairing the outstanding formability as in curability and workability and the high mechanical strength inherent in the radically polymerization resin. We have consequently found that a resin composition comprising (A) an unsaturated ester obtained by the reaction of an epoxy compound possessing at least two epoxy groups in the molecular unit thereof with an unsaturated monobasic acid and optionally a polybasic acid and/or (B) an unsaturated ester obtained by the reaction of a compound possessing at least two phenolic hydroxyl groups in the molecular unit thereof with a compound represented by the formula (1): ##STR2## wherein R.sup.1 and R.sup.2 independently stand for hydrogen atom or methyl group, and possessing an epoxy group and a radically polymerizable unsaturated bond in the molecular unit thereof, (C) a compound possessing at least one maleimide group in the molecular unit thereof, and optionally (D) a polymerizable cross-linking agent meets the purpose mentioned above, (Japanese patent application No. SHO61(1986)-247,617).
Indeed this resin composition manifests the outstanding properties mentioned above without any problem so long as it is put to use within a relatively short period of time after the formulation thereof. When this resin composition is kept in protracted storage, however, there is entailed a disadvantage that even in the absence of a polymerization initiator, the composition suffers from a conspicuous increase of viscosity or undergoes gelation at room temperature and becomes hardly workable.
We further continued various studies for the purpose of developing a resin capable of producing cured articles of high stability at elevated temperatures without any sacrifice of the outstanding formability such as curability and workability and the high mechanical property inherent in the radical polymerization type resin. We have consequently found that a vinyl ester resin composition comprising (A') an unsaturated ester compound represented by the formula (2): ##STR3## wherein R.sup.3 's are independently selected from the group consisting of ##STR4## wherein R.sup.5 and R.sup.6 independently stand for hydrogen atom or methyl group, and hydrogen atom, at least one of the R.sup.3 's stand for ##STR5## wherein R.sup.5 and R.sup.6 have the same meanings as defined above, R.sup.4 's stand for one member selected from the group consisting of hydrogen atom, halogen atoms, methoxy group, and alkyl groups of 1 to 5 carbon atoms, and m stands for 0 or an integer in the range of 1 to 10, and/or (B') an unsaturated ester compound represented by the formula (3): ##STR6## wherein R.sup.7 's are independently selected from the group consisting of ##STR7## wherein R.sup.5 and R.sup.6 have the same meanings as defined above, and hydrogen atom, at least one of R.sup.7 's stands for ##STR8## wherein R.sup.5 and R.sup.6 have the same meanings as defined above, R.sup.8 's stand for at least one member selected from the group consisting of hydrogen atom, halogen atoms, methoxy group, and alkyl groups of 1 to 5 carbon atoms, and n stands for 0 or an integer in the range of 1 to 10, providing that X stands for a divalent organic group selected from the group consisting of ##STR9## wherein n is 0 or X's stand for divalent organic groups independently selected from the group consisting of ##STR10## --CH.sub.2 -- where n is an integer in the range of 1 to 10, and (C') a polymerizable cross-linking agent meets the purpose mentioned above (Japanese patent application No. SHO 61(1986)-75,420, No. SHO 61(1986)-131,767, and No. SHO 61(1986)-156,132).
This resin composition manifests the outstanding properties mentioned above without any trouble so long as it is put to use within a short span of time after the preparation thereof. When this resin composition is kept in protracted storage, however, there is encountered a disadvantage that even in the absence of a polymerization initiator, the composition suffers from a remarkable increase of viscosity or undergoes gelation at room temperature and becomes hardly workable.
A method for improving, by the addition of a thiuram compound, the storage stability of a thermosetting resin obtained by dissolving the reaction product of an epoxy resin with an .alpha.,.beta.-ethylenic monocarboxylic acid in a polymerizing olefinic compound has been known to the art (U.S. Pat. No. 4,129,609). By the addition of the thiuram compound, however, the aforementioned thermosetting resin does not acquire any satisfactory storage stability.
Another object of the present invention, therefore, is to provide a resin composition with highly satisfactory storage stability.
Another object of this invention is to provide a method for imparting storage stability to a resin composition which excels in formability and produces cured articles excelling in stability at elevated temperatures and in mechanical strength.